🐛 [BUG] - Remove right padding on RiverAccordion #1500

Describe the bug

Came up in https://github.com/github/brand-experience/issues/605 (internal)

Not sure if it's a bug on intended, but it feels a bit odd that RiverAccordion has that extra space on the right side with the gridline variant.

Reproduction steps

1. Go to the docs
2. Notice the extra space on the right

Expected behavior

The visual should move right to the edge so that both columns take up exactly 50%.

Screenshots

Urgency

Not too urgent. Current workaround is to override it with:

<RiverAccordion variant="gridline" className="RiverAccordion">

.RiverAccordion {
  --half-column: 50%;
}

I guess the problem is that gap is set to 0, but --half-column subtracts half of the gap, causing the 2 columns to not fill the available space.